1. Field of the Invention
The present invention relates to a liquid crystal display device which improves the problem of an afterimage, and a method for driving the same.
2. Description of the Related Art
When a user stops using and turns off a conventional liquid display device, the device is shut down without any operation for clearing the image on the display. The supply of various signals (scanning line driving signals, data line driving signals, or the like) to the liquid crystal display panel is stopped, and the paths for externally discharging the charge from the liquid crystal capacitance of the liquid crystal display panel is blocked. The charge then gradually decreases because of self-discharge, so that the displayed image is gradually cleared.
However, when the charge is kept in the liquid crystal capacitance for a long time, an afterimage may be produced, the quality of the display and the reliability in use for years may be degraded.
The mechanism for causing the afterimage will be explained.
FIGS. 7A and 7B are schematic diagrams of a unit pixel of the liquid crystal panel in the display device. According to the basic structure of the unit pixel, liquid crystal is enclosed between two electrodes, and a voltage corresponding to the image signal is applied between the electrodes so that the orientation of the liquid crystal molecules is changed. The transmittance of light is thus controlled so as to provide a desired gradation.
When manufacturing the unit pixel, a small amount of ionizable material P may be mixed between the electrodes in step of enclosing the liquid crystal material (see FIG. 7A). Even when the ionizable material P is enclosed, as long as an ideal alternating signal is applied between two electrodes, the material P is not stacked on the electrodes, and does not affect the transmittance of light, that is, the orientation of the liquid crystal molecules.
However, the alternating voltage, which is actually applied to both the electrodes, always contains the component of a direct current. The voltage of the direct current is applied between the electrodes, the ionizable material P is drawn to one of the electrodes because of the characteristics of ion, and is stacked on the electrode as shown in FIG. 7B. When the ionizable material P is stacked on the electrode while the alternating current representing the video image is applied between the electrodes, the voltage applied to the liquid crystal is affected by the ionizable material P stacked on the electrode, and the orientation of the liquid crystal molecules is controlled by the different voltage. When a large amount of ionizable material P is stacked on the electrode, the voltage applied to the liquid crystal is significantly changed, so that the brightness significantly differs from other pixels on which no ionizable material is stacked. This is visually recognized as the afterimage.
To prevent the afterimage, the liquid crystal device disclosed in Japanese Patent No. 2655328 detects the point at which the power is turned off, and directs a power maintaining circuit to maintain turning on switching elements, corresponding to pixel electrodes, for a specified time. Thus, discharging paths are kept opened so that the charge stored in the liquid crystal capacitance can be discharged, and then the liquid crystal display device is turned off.
In general, the sequence of turning off the power to the liquid crystal display device comprises turning off a backlight to prevent the display of the distorted image on the liquid crystal display panel, subsequently stopping sending input signals such as a synchronizing signal, and a video signal, and subsequently turning off the power supply.
While the conventional liquid crystal display device discharges the charge from the liquid crystal capacitance after detecting that the power supply has been turned off, the liquid crystal is charged up in a short time from the stopping the input signals such as the synchronizing signal to the turning-off of the power supply. That is, the direct current is applied to the liquid crystal, decreasing the long-term reliability of the liquid crystal material, and causing the afterimage.
Further, FA (Factory Automation), and monitors, which have been developed in recent years, include a plurality of devices some of which have the liquid crystal display devices. The power to all the devices may be supplied from the same power source. In this case, only the liquid crystal display device cannot be turned off. Therefore, when finishing using the liquid crystal display device, only the input signals are stopped, and the power supply to the liquid crystal display device is not turned off.
Even when the conventional liquid crystal display device, which can discharge the charge from the liquid crystal capacitance at the time of turning off the power supply, is applied to the FA, the charge stored in the liquid crystal capacitance cannot be forcibly discharged.
Therefore, until the charge stored in the liquid crystal capacitance disappears by self-discharge, the direct voltage is continuously applied to the liquid crystal. The ionizable material in the liquid crystal is stacked on the electrodes, thus causing the afterimage.
As mentioned above, even in the conventional liquid crystal display device disclosed in Japanese Patent No. 2655328, as the power supply is repeatedly turned on and off, the ionizable material is stacked on the electrode, the afterimages, or stains are produced, decreasing the life of the liquid crystal, and the reliability in use for years.